Abstract

In functional MRI it is desirable for the blood-oxygenation level dependent (BOLD) signal to be localized to the tissue containing activated neurons rather than the veins draining that tissue. This study addresses the dependence of the specificity of the BOLD signal - the relative contribution of the BOLD signal arising from tissue compared to venous vessels - on magnetic field strength. To date, studies of specificity have been based on models or indirect measures of BOLD sensitivity such as signal to noise ratio and relaxation rates, and assessment has been made in isolated vein and tissue voxels. The consensus has been that ultra-high field systems not only significantly increase BOLD sensitivity but also specificity, that is, there is a proportionately reduced signal contribution from draining veins. Specificity was not quantified in prior studies, however, due to the difficulty of establishing a reliable network of veins in the activated volume. In this study we use a map of venous vessel networks extracted from 7 T high resolution Susceptibility-Weighted Images to quantify the relative contributions of micro- and macro-vasculature to functional MRI results obtained at 3 and 7 T. High resolution measurements made here minimize the contribution of physiological noise and Independent Component Analysis (ICA) is used to separate activation from technical, physiological, and motion artifacts. ICA also avoids the possibility of timing-dependent bias from different micro- and macro-vasculature responses. We find a significant increase in the number of activated voxels at 7 T in both the veins and the microvasculature - a BOLD sensitivity increase - with the increase in the microvasculature being higher. However, the small increase in sensitivity at 7 T was not significant. For the experimental conditions of this study, our findings do not support the hypothesis of an increased specificity of the BOLD response at ultra-high field.

Verification of the accuracy of the normalization 7 T → SWI space and 3 → 7 T (SWI) space for a typical subject and illustration of the corresponding vein map. For the illustration only, the boundaries of the veins (rather than the vein masks themselves) are shown, overlaid in cyan. These were generated with the contour function of CorelDraw (Corel Corporation, Ottawa, ON, Canada). Bottom row: zoomed depiction of the hand area.

Exemplified single subject illustration (radiological convention). (A) Functional image with vessels (cyan) thresholded IC map and anatomical VOI (green) overlaid. (B) As in (A), but with no thresholding applied to IC map. A zoomed representation of the target area is also illustrated.